Expert Report

Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future (2012)

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Tide gages show that global sea level has risen about 7 inches during the 20th century, and recent satellite data shows that the rate of sea-level rise is accelerating. As Earth warms, sea levels are rising mainly because: (1) ocean water expands as it warms; and (2) water from melting glaciers and ice sheets is flowing into the ocean. Sea-level rise poses enormous risks to the valuable infrastructure, development, and wetlands that line much of the 1,600 mile shoreline of California, Oregon, and Washington. As those states seek to incorporate projections of sea-level rise into coastal planning, they asked the National Research Council to make independent projections of sea-level rise along their coasts for the years 2030, 2050, and 2100, taking into account regional factors that affect sea level.

Sea level along the U.S. west coast is affected by a number of factors, including climate patterns such as the El Niño, effects from the melting of modern and ancient ice sheets, and geologic processes, such as plate tectonics. Regional projections for California, Oregon, and Washington show a sharp distinction at Cape Mendocino in northern California. South of that point, sea-level rise is expected to be very close to global projections. However, projections are lower north of Cape Mendocino because the land is being pushed upward as the ocean plate moves under the continental plate along the Cascadia Subduction Zone. However, an earthquake magnitude 8 or larger, which occurs in the region every few hundred to 1,000 years, would cause the land to drop and sea level to suddenly rise.

Key Messages

Melting of land ice is now the largest component of global sea-level rise (about 65%), largely because ice loss rates are increasing.

Vertical land motions caused by plate tectonics and the ongoing response of the Earth to the disappearance of North American ice sheets have a significant impact on sea-level rise along the Washington, Oregon, and California coasts.

Sea level along the California coast south of Cape Mendocino is projected to rise 4-30 cm (2-12 in) by 2030, relative to 2000 levels, 12-61 cm (5-24 in) by 2050, and 42-167 cm (17-66 in) by 2100. These projections are close to global sea-level rise projections.

For the Washington, Oregon, and California coasts north of Cape Mendocino, sea level is projected to change between -4 cm (-2 in) (sea-level fall) and +23 cm (9 in) by 2030, -3 cm (-1 in) and +48 cm (19 in) by 2050, and 10-143 cm (4-56 in) by 2100. These values are lower than projections further north.

An earthquake magnitude 8 or greater along the Cascadia Subduction Zone would suddenly raise sea level along parts of the coast by an additional 1-2 meters (3-7 feet) over projected levels north of Cape Mendocino.

Uncertainties grow as the projection period lengthens. Confidence in the projections is high for 2030 and perhaps 2050. By 2100, we are confident only that the value will fall within the uncertainty bounds.

Most coastal damage is caused by the confluence of large waves, storm surges, and high astronomical tides during a strong El Niño.

Some models predict a northward shift in North Pacific storm tracks, and some observational studies report that largest waves are getting higher and winds are getting stronger. Observational records are not long enough to confirm whether these are long-term trends.

Even if storminess does not increase in the future, sea-level rise will magnify the adverse impact of storm surges and high waves on the coast.

Storms and sea-level rise are causing coastal cliffs, beaches, and dunes to retreat at rates from a few cm/yr to several m/yr. Cliffs could retreat more than 30 m (about 100 feet) by 2100.

Wetlands are likely to keep pace with sea level until 2050. Their survival until 2100 depends on maintaining elevation through high sedimentation, room to move inland, or uplift.